1. Field of the Invention
The present invention relates to an electroluminescent device using an organic compound and, more particularly, to an electroluminescent device having high luminescence efficiency and a long luminescence life achieved by using a metal coordination compound as a luminescent material.
2. Related Background Art
A lot of studies is conducted on organic electroluminescent (EL) devices for practical use, which are electroluminescent devices having high-speed response and high efficiency. FIGS. 1A and 1B show basic constructions of such electroluminescent devices (for example, see Macromol. Symp. 125, 1-48 (1997)).
As shown in FIGS. 1A and 1B, a typical organic EL device is ordinarily constituted by a transparent electrode 14 on a transparent substrate 15, a metal electrode 11, and a plurality of organic film layers provided between the transparent electrode 14 and the metal electrode 11.
Referring to FIG. 1A, the organic layers are a light-emitting layer 12 and a hole transport layer 13. ITO or the like having a large work function is used as the transparent electrode 14. Thus, there are provided good characteristics of hole injection from the transparent electrode 14 into the hole transport layer 13. A metal material such as aluminum, magnesium or an alloy of aluminum and magnesium having a small work function is used as the metal electrode 11 to obtain good characteristics with respect to injection of electrons to the organic layers. These electrodes typically have a film thickness of 50 to 200 nm.
For the light-emitting layer 12, an aluminum quinolinol complex having electron-transport and luminescence characteristics, typically Alq3 shown in FIG. 2A or the like is used. For the hole transport layer 13, an electron donative material such as triphenyl diamine derivative, typically α-NPD shown in FIG. 2A or the like is used.
The device constructed as described above exhibits a rectification characteristic. When an electric field is applied such that the metal electrode 11 functions as a cathode and the transparent electrode 14 functions as an anode, electrons are injected from the metal electrode 11 into the light-emitting layer 12 while holes are injected from the transparent electrode 14.
The injected holes and electrons recombine with each other in the light-emitting layer 12 to cause excitons, thereby emitting light. At this time, the hole transport layer 13 functions as an electron blocking layer. Therefore the efficiency of recombination at the interface of the light-emitting layer 12 and the hole transport layer 13 is increased. The luminescence efficiency is thereby increased.
Referring to FIG. 1B, an electron transport layer 16 is further provided between the metal electrode 11 and the light-emitting layer 12 shown in FIG. 1A. When an electron transport layer is additionally provided independently in this manner to obtain a more effective carrier blocking structure, more efficient luminescence can be effected in the luminance layer. As a material for the electron transport layer 16, an oxadiazole derivative, or Alq3 or Bphen shown in FIGS. 2A and 2B, for example, may be used.
When a typical organic EL device emits light, fluorescence takes place which is caused by transition from singlet excitons to the ground state in molecules having emission centers. On the other hand, studies are being made on devices not using fluorescence via singlet excitons but using phosphorescence via triplet excitons.
The followings are typical references made public about the results of such studies.    Reference 1: D. F. O'Brien et al., “Improved energy transfer in Electrophosphorescent device”, Applied Physics Letters Vol. 74, No. 3, p. 422 (1999)    Reference 2: M. A. Baldo et al., “Very high-efficiency green organic light-emitting devices based on Electrophosphorescence”, Applied Physics Letters Vol. 75, No. 1, p. 4 (1999)
Devices mainly used in these references are constructed by four organic layers as shown in FIG. 1C. This device is formed by stacking, from the anode side, a hole transport layer 13, a light-emitting layer 12, an exciton diffusion prevention layer 17, and an electron transport layer 16. Materials used in the device are carrier transport materials and phosphorescent materials shown in FIGS. 2A and 2B. The followings are the meanings of acronyms shown in FIGS. 2A and 2B.    Alq3: aluminum-quinolinol complex    α-NPD: N4,N4′-Di-naphthalen-1-yl-N4,N4′-diphenyl-biphenyl-4,4′-diamine    CBP: 4,4′-N,N′-dicarbazole-biphenyl    BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline    Bphen: 4,7-diphenyl-1,10-phenanthroline    PtOEP: platinum-octaethylporphylin complex    Ir(ppy)3: iridium-phenylpyridine complex
Devices capable of obtaining high efficiency according to the above-mentioned two references are those formed by using α-NPD in the hole transport layer, Alq3 in the electron transport layer, and BCP in the exciton diffusion prevention layer, and using CBP as a host material in the light-emitting layer and mixing therein a phosphorescent material such as Ir(ppy)3 or PtOEP at a concentration of about 6% in the material of the light-emitting layer.
In recent years, phosphorescent materials have attracted particular attention because a high luminescence efficiency can be expected in theory. That is, excitons generated by carrier recombination consist of singlet excitons and triplet excitons and the ratio in probability of their occurrence is 1:3. The conventional organic EL devices emit light by utilizing fluorescence caused by transition from singlet excitons to the ground state. The luminescence efficiency of such fluorescence in theory is 25% with respect to the number of generated excitons, and this value is a theoretical upper limit. However, if phosphorescence from excitons generated from triplets is used, at least a treble yield can be expected in theory. Further, if transition by intersystem crossing from higher singlets in energy to triplets is included, a quadruple luminescence yield, i.e., 100% luminescence yield, can be expected in theory.
While the organic EL devices using phosphorescence have a problem of luminescence efficiency, they also have the problem of deterioration of luminescence at the time of energization. The cause of luminescence deterioration of electrophosphorescent devices is presently uncertain, but it is thought that it is caused by reaction with neighboring materials, formation of an excited multimer, change in molecular microstructure, influence of impurities, structural change of neighboring materials, etc., which occur because the triplet life is ordinarily longer than the singlet life by three or more orders of magnitude, and the molecules are maintained in a high-energy state.
Phosphorescent devices are studied by many groups of researchers. Metal coordination compounds that are employed in the studies have iridium as the central metal.
The following Reference 3 reports as this example.    Reference 3: S. Lamansky et al., “Synthesis and Characterization of Phosphorescent Cyclometalated Iridium complex”, Inorg. Chem. 40, p. 1704, (2001)
In general, a material capable of effecting phosphorescence with high efficiency is a compound that has a central metal of a relatively large atomic weight. Phosphorescence is caused by transition from excited triplets to ground singlets. Such transition is a forbidden transition in the case of ordinary organic materials. In some metal coordination compound using a heavy-atom metal, however, this forbiddance is eliminated by “heavy-atom effect” so that the transition is allowed to occur resulting in strong phosphorescence occurrence.
An example of such a metal coordination compound is a metal coordination compound using iridium metal. However, a metal coordination compound using a heavy-atom metal such as an iridium coordination compound, of course has a large molecular weight. Therefore, there is a problem that thermal decomposition of the iridium coordination compound occurs during film deposition in fabrication of an organic EL device. If decomposition occurs, a decomposition product is mixed in the formed light-emitting layer, thereby causing reduction of luminescence life of the luminescent device and considerable variation in the devices at the time of mass production.
Typical iridium metal coordination compounds are, for example, compounds having molecular structures represented by Nos. 31 to 35 in FIG. 3. In compound 31, three phenylpyridine ligands are coordinated. In compound 33, two phenylpyridine ligands are coordinated. Compounds 31 and 33 have molecular weights of 655 and 600, respectively. On the other hand, Alq3 shown in FIG. 2A has a molecular weight of 459 smaller than those of the above-described iridium coordination compounds.
These compounds were vacuum-deposited at the same deposition rate and residues in the deposition boat were analyzed. The results of the analysis showed that decomposition does not occur in the case of Alq3. The results also shows that, in the case of iridium coordination compound deposition for a long time, decomposition compounds of 0.1 to 3% were produced. It is thereby understood that there is a possibility of the iridium coordination compound decomposing in the deposition boat under severe conditions, such as those under which a number of devices are made in mass production, to cause an increase of impurity concentration in the device, which may adversely affect the device performance and the production stability.
Further, it is likely that a coordination compound having a ligand with a longer conjugate length is produced to allow light emission with longer wavelength. Compounds 34 and 35 are shown in FIG. 3 as an example of a compound modified in this manner. In this case, the longer ligand results in a larger molecular weight, and a higher sublimation temperature, which makes the deposition more difficult and adversely affects the productivity.
As a coordination compound with a reduced molecular weight, hetero-ligand compounds 33 to 35 have been proposed based on ternary coordination compounds such as compounds 31 and 32 shown in FIG. 3. However, the illustrated acetylacetonate is lower in thermostability than ternary coordination compounds, and a problem of decomposition of the compound occurs.
Phosphorescent materials also have a problem of deterioration by a chemical reaction in the device at the time of energization in addition to the above-mentioned impurity problem.
There is also a case where a rhenium complex is used in a light-emitting layer of an electroluminescent device, which is disclosed in the following reference, in which several rhenium complexes are described. However, there is a demand for further development of novel luminescent materials.    Reference 4: Chinese Patent Publication No. 1084134C
As described above, the above-described metal coordination compounds for use in phosphorescent devices have the problems described so far. Therefore, there is a need for materials having higher luminescence efficiency and higher chemical stability.